The present invention relates generally to microactuators and more specifically to an electromagnetic microactuator having a ferromagnetic substrate and a method for making same.
There are a variety of microactuators in the art based on electrostatic, thermal mechanical, piezoelectric, shape memory alloy or electromagnetic actuation principles. For automotive applications, microactuators are required to have large displacement (in the tens of micrometers), wide operational temperatures (from minus 40 to plus 125 degrees Celsius) and low operational voltages (12V). Under such requirements, electromagnetic actuation is the best choice. An electromagnetic microactuator includes an inductive component that generates a magnetic flux and a magnetic core to guide the magnetic flux. Construction of electromagnetic microactuators include the use of AZ400 series positive photoresists or photosensitive polyimide to form the plating mold. However, the AZ400 phtotoresist has an aspect ratio of less than 3 and poor planarization.
Silicon wafers are used as a substrate for electromagnetic microactuators. Using a silicon wafer as the substrate requires a long processing time. Five hours is required for a 300 micrometer deep cavity, and electroplating of the bottom return core takes 10 hours for 300 micrometer thick permalloy. There is also a large thermal expansion coefficient mismatch among the copper, permalloy and silicon components. The thermal expansion coefficients of copper, permalloy and silicon are 17, 15 and 3 ppm/degree Celsius, respectively. The differences in thermal expansion may cause difficulties in device fabrication and have a detrimental effect on device performance.
By way of instructive example, FIG. 1 shows a prior art silicon-type electromagnetic microactuator 100 fabricated with a silicon wafer substrate 102. The electromagnetic microactuator 100 further includes a center core 104, a peripheral core 106, a spiraling copper coil 108 and terminal pads 110 connected to the coil ends. The silicon substrate 102 has a cavity 112 formed therein whereat is located a flux return path core 114 fabricated from nickel-iron. A silicon dioxide layer 116 is formed on the silicon substrate 102 before application of the center core 104, the peripheral core 106, the copper coil 108 and the terminal pads 110. The center core 104, the peripheral core 106, the copper coil 108 and the terminal pads 110 are formed on the silicon dioxide layer 116 with microelectroforming techniques. The center and peripheral cores are fabricated from nickel/iron. An SU-8 masking material 118 is also used. A ferromagnetic plate-shaped armature 120 is disposed adjacent the electromagnetic microactuator 100. When electrical current is run through the copper coil 108, magnetic flux is generated in the center and peripheral cores in cooperation with the return flux path core 114, resulting in an attractive magnetic force FM applied to the armature 120. This magnetic force causes the armature to move toward the electromagnetic microactuator 100, overcoming opposed biasing (as for example by a return spring).
The present invention is an electromagnetic microactuator having a ferromagnetic substrate and a method for making same, which provides better planarization and aspect ratios than that of the prior art, and further simplifies fabrication.
The method of fabrication of an electromagnetic microactuator according to the present invention includes a ferromagnetic substrate (base), preferably steel, and a plurality of mask deposited layers.
A first processing step includes a spin coat of spin-on-glass upon the substrate, patterned using a contact mask. A second processing step includes depositing a seed layer of titanium-tungsten-gold, patterned using an anchor mask. A third processing step includes depositing a sacrificial layer of chromium-aluminum, patterned using a plating mask. A fourth processing step includes a spin coat of an SU-8 photoresist first mold layer, patterned using a copper coil mask. A fifth processing step includes electroplating with copper. A sixth processing step includes stripping the SU-8 photoresist first mold layer with plasma etching. A seventh processing step includes stripping the sacrificial layer by an etching solution, an eighth processing step includes a spin coat of an SU-8 photoresist second mold layer, patterned using a core mask, which intersticially fills and covers the coil. A ninth and final processing step includes electroplating center and peripheral nickel-iron cores.
Accordingly, it is an object of the present invention to provide a method for fabrication of an electromagnetic microactuator, which simplifies fabrication by utilization of a ferromagnetic substrate.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.